Method for production of sulfhydryl compounds

ABSTRACT

An improved process is disclosed for production of sulfhydryl-terminated compounds by reaction of an allene and hydrogen sulfide initiated with ultraviolet light of wavelength above about 2600 Angstroms to obtain a colorless product of improved purity. The invention also includes the polymerization of an unsaturated hydrocarbon with the sulfhydryl-terminated product to prepare a sulfhydryl-terminated polythioether. These compounds are useful as chemical intermediates and precursors of elastomeric sealants, such as architectural sealants.

This is a division, of application Ser. No. 535,631, filed Dec. 23,1974, now U.S. Pat. No. 3,992,289.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to improvements in the manufacture ofsulfhydryl-terminated compounds useful as chemical intermediates andprecursors of polythioether elastomeric sealants. By the method of theinvention, products of improved purity are obtained by the addition ofhydrogen sulfide to an allene compound. Subsequent addition of thesulfhydryl compound to a polyunsaturated olefin and/or an acetyleneproduces sulfhydryl-terminated polythioethers of improved purity andcolor. Such polythioethers are useful as a base polymer for sealants.

1. Description of the Prior Art

The ultraviolet light initiated free radical addition of hydrogensulfide to allene in the liquid phase is known to producesulfhydryl-terminated compounds such as mercaptans andsulfhydryl-terminated thioethers. U.S. Pat. No. 3,488,270 to Griesbaumet al. discloses the liquid phase addition of hydrogen sulfide to alleneinitiated with ultraviolet light. The reaction is conducted in quartzwhich transmits high energy ultraviolet light, i.e. ultraviolet light ofwave length below 2600 Angstroms as well as low energy, long wavelengthultraviolet light, i.e. ultraviolet light of wavelength above about 2600Angstroms. The reaction product is yellow and is usually contaminatedwith elemental sulfur. The formation of sulfur is accompanied byhydrogen evolution which may produce sufficient pressure to burst thereaction vessel. It has not been found feasible by conventionalpurification techniques to remove the discoloration. This disadvantageis particularly serious since the principal reaction product,1,3-propane dithiol, is the precursor of sulfhydryl-terminatedpolythioethers which cure to valuable elastomeric sealants. Thus whenthe discolored dithiol, is reacted with an acetylenically and/orpolyethylenically unsaturated compound in the presence of ultravioletlight as initiator according to known techniques (see U.S. Pat. Nos.3,592,798 and 3,717,618 to Oswald and coassigned U.S. Ser. No. 501,716,filed Aug. 29, 1974, entitled "Polythioether Sealant Compositions" to B.F. Dannels), the sulfhydryl polythioether obtained is also discoloredand of unsatisfactory curing characteristics and, hence is commerciallyunacceptable.

U.S. Pat. Nos. 2,398,481 and 2,411,983 to W. E. Vaughan et al. and U.S.Pat. No. 2,873,239 to W. R. Nummy et al. disclose the liquid phaseaddition of hydrogen sulfide to unsaturates initiated by ultravioletlight of wavelength above 2900-3000 Angstroms. The references neitherrelate to use of reactants having the distinctive allene unsaturatedgroup, i.e. two carbon to carbon double bonds attached to the samecarbon atom, nor teaches substantial conversion to sulfhydryl-terminatedproducts in absence of a photosensitizing reagent.

U.S. Pat. No. 3,412,001 to J. R. Edwards discloses liquid phase reactionof hydrogen sulfide and olefins in the presence of ultraviolet lightfiltered through Pyrex, i.e. ultraviolet light of wavelengthsubstantially above about 2800-2900 Angstroms. However, the patent doesnot relate to reaction of olefins containing the distinctive allenelinkage and, moreover, requires limiting the reaction to conversion ofno more than 22% of olefin starting material.

SUMMARY OF THE INVENTION

The invention relates to a novel improvement in the preparation ofsulfhydryl-terminated compound by reaction of an allene compound withhydrogen sulfide which improvement comprises carrying out the reactionin the presence of ultraviolet light of wavelength substantially aboveabout 2600 Angstroms to obtain a product substantially free of color andelemental sulfur.

The allene compound employed is either allene or a substituted allene.When allene is used, principal sulfhydryl-terminated product is1,3-propanedithiol. When a substituted allene is employed, the principalproduct is generally the corresponding substituted 1,3-propanedithioland/or substituted 1,2-propanedithiol.

In a preferred embodiment of the invention the colorlesssulfhydryl-terminated product is polymerized with an ethylenic oracetylenic compound to obtain a substantially colorlesssulfhydryl-terminated thioether polymer curable to a polythioethersealant. The polymerization is also preferably carried out in thepresence of ultraviolet light of wavelength substantially above 2600Angstroms.

It was highly surprising to discover that ultraviolet light devoid ofits high energy short wavelength bands would effectively initiate thereaction of the allene compound and hydrogen sulfide, avoid hydrogenevolution and provide a product devoid of undesirable color andelemental sulfur. The elimination of elemental sulfur and hydrogenformation in the reaction of the allene compound and hydrogen sulfide bythe improved process of the invention is highly beneficial. The sulfurand hydrogen formation consumes hydrogen sulfide charged to the reactionand hydrogen evolution may produce dangerously high pressure in thereaction vessel.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS THEREOF

The reaction of the allene compound, i.e. allene or a substitutedallene, and hydrogen sulfide is carried out substantially in accord withprior art reaction techniques except that the ultraviolet light incidenton the reaction mixture is filtered to exclude ultraviolet light ofshort wavelength, i.e. wavelength of about 100 to about 2600 Angstroms.Generally ultraviolet light of the desired wavelength above 2600Angstroms can be obtained by filtering broad spectrum ultraviolet light,i.e., of wavelength of about 100-3800 Angstroms, through a vitreousmaterial which is transparent to ultraviolet of the desired longwavelengths but opaque to ultraviolet light of wavelengths of 100-2600Angstroms. Suitable vitreous materials for use in the invention includeglasses such as the proprietary glass compositions manufactured by theCorning Glass Co., for example, Corex (No. 9700), Corex D, ChemicalPyrex (No. 7740), and Nonex (No. 7720) as well as lead glass and sodalime glass. Corex (No. 9700) which transmits ultraviolet light ofwavelengths above about 2600 Angstroms and Chemical Pyrex whichtransmits ultraviolet light above about 2800 Angstroms are preferredfiltering materials for obtaining ultraviolet light of the desired longwavelengths. The preferred materials contain 80-81% SiO₂, 13% B₂ O₃,4-5% NA₂ O and 2% Al₂ O₃.

The ultraviolet transmission characteristics of various glasses (at 1 mmthickness) suitable for use in the invention are compared with those ofglasses e.g. quartz and Vycor, which transmit undesirable shortwavelength ultraviolet radiation in the section entitled "TransmissionCharacteristics of Quartz and Various Glasses" of "The General ElectricFused Quartz Catalog," Q-3, General Electric Co., 1952, the pertinenttechnology of which is incorporated herein by reference.

The reaction mixture obtained by reaction of the allene compound andhydrogen sulfide according to the invention is colorless and devoid ofelemental sulfur. Generally only fractional distillation of the mixture,advantageously effected at diminished pressure, e.g. 0.1 to 100 mm, isrequired to provide a sulfhydryl-terminated product which reacts with apolyethylenically or acetylenically unsaturated compound to provide asubstantially colorless sulfhydryl-terminated polythioether. This resultis in unexpected contrast to that obtained by the prior art methodutilizing ultraviolet light containing short wavelengths wherein theproduct, even after purification by distillation and absorptionchromatography on alumnia, remains colored and produces, on reactionwith the unsaturated compound, a yellow, unattractive, unacceptablepolythioether.

The allene compound utilized in the present process is distinguishedfrom ordinary olefins, such as ethylene, propylene and the like, inhaving two carbon-to-carbon double bonds attached to the same carbonatom. Allene compounds for use in general in the invention correspond tothe general formula: ##STR1## wherein R₁ R₂, R₃ and R₄ independentlyrepresent hydrogen and hydrocarbon radicals of 1 to 20 carbon atoms. Thehydrocarbon radicals can be saturated straight-chain and branched chainalkyl radicals, cycloalkyl radicals, aryl radicals, aralkyl radicals andalkaryl radicals and R₁ and R₃, when alkyl, can be joined together toform a cycloaliphatic ring. The R₁ R₂ R₃ and R₄ radicals may be furthersubstituted with conventional inert substituents such as halogen, e.g.chlorine, bromine and fluorine, carboalkoxy, i.e. ester substituents,carbonyl, i.e. keto, substituents and the like.

Typical allene reactants for use in general in the invention include thefollowing: ##STR2##

If desired, mixtures of these and equivalent allenes including homologsand isomers thereof can be charged as the allene compound reactant.

All of the foregoing representative compounds are known or are readilyprepared by conventional synthetic techniques described in S. Patai, Ed."The Chemistry of Alkenes," Interscience Publishers, 1964, pages659-661, 762-3, 770, 1030-1060 and T. F. Rutledge, "Acetylenes andAllenes," Rheinhold Book Corporation, 1969, pages 4-31. The pertinentsubject matter of these references is incorporated herein by reference.

Preferably the allene compound used is a lower aliphatic allene i.e.allene substituted with saturated alkyl groups of one to six carbonatoms or hydrogen. Preferably also an allene compound wherein the allenelinkage is terminal, i.e. wherein R₃ and R₄ are hydrogen in the aboveformulas, is employed in the practice of the invention. Allene itselfprovides an especially good result. Frequently allene contains smallamounts (up to about 0.5 to 10 mole percent) of methylacetylene which isalso capable of reacting with hydrogen sulfide under the conditions ofthe present improved process. However, it is found that the presence ofmethylacetylene in the allene starting material in no way interfereswith the beneficial excellent result obtained in the process.Accordingly such allene, can advantageously be used in the presentprocess.

The reaction of hydrogen sulfide and the allene compound is carried outin the liquid phase i.e. under sufficient pressure to maintain thehydrogen sulfide and organic reactants and products in the liquid phase.In general the reaction may be effected at temperatures in the range ofabout -100° to about +95° C. at reaction pressures ranging from aboutatmospheric (0 p.s.i.g.) to about 3000 p.s.i.g. Preferable the reactionis carried out at about -70° C to about +90° C, at pressures rangingfrom atmospheric to about 1300 p.s.i.g., especially at about -20° C toabout 70° C at pressures about 60 to about 400 p.s.i.g. If desired, thereaction can be carried out efficiently at low pressures ranging fromabout atmospheric pressure to 60 p.s.i.g. at reaction temperatures ofabout -70° C to -40° C.

The reaction of the hydrogen sulfide with the allene compound isexothermic. The temperature in the reaction vessel can be convenientlycontrolled by conventional cooling techniques. For example a coolingcoil can be disposed within the reaction mixture or a cooling jacket cansurround the reaction vessel. Also streams of air or nitrogen flowedalong the outside walls of the vessel are useful in cooling the reactionvessel or for removing the heat generated by the ultraviolet lightsource.

The source of ultraviolet light employed in carrying out the presentprocess include sunlight, a carbon arc, a mercury vapor lamp, a tungstenarc or Kronmeyer lamp. Advantageously the light source is disposedoutside the reaction vessel which is constructed of the vitreousmaterial employed in the invention to filter the ultraviolet light.Alternatively there is used a reaction vessel which is constructedpredominantly of stainless steel, glass-lined stainless steel or otherlight-opaque material inert to the reactants and products of the presentprocess but which contains a window or, advantageously a light well,constructed of the above described vitreous, ultraviolet filteringmaterial.

If desired, a quartz well equipped with a removable filter insert of thedesired vitreous material is used. When a reaction vessel containing alight well is employed the ultraviolet light source, advantageously amercury vapor lamp, is disposed within the well during the reaction. Aswill be apparent to those skilled in the art, the reaction vessel usedshould be capable of withstanding superatmospheric pressure when suchpressure is employed in carrying out the reaction.

The molar proportion of hydrogen sulfide to the allene compound employedcan be from about 1:1 to about 20:1 and is preferably about 1.5:1 toabout 15:1.

The molar ratio of hydrogen sulfide to the allene compound is animportant feature of the process. This is so since the principalsulfhydryl-terminated product (i.e. 1,3-propanedithiol or substituted1,3-propanedithiol is generally accompanied by a complex mixture ofother sulfhydryl-terminated compounds including sulfhydryl-terminatedthioethers and polythioethers with the yield of the desired dithiolbeing dependent upon the relative proportion of reactants charged.Typically the reaction of allene and hydrogen sulfide in preferredratios produces a crude product mixture containing about 50 to 70% ofthe principal desired product, 1,3-propanedithiol, about 30 to 50% ofsulfhydryl-terminated thioethers containing one, two, three, four ormore thioether linkages (i.e. C-S-C) per molecule and about 0 to 5% of1,2-propanedithiol. An especially good result is obtained in the presentprocess employing hydrogen sulfide and the allene compound in a molarratio of about 2.0:1 to 10:1.

The present improved addition of hydrogen sulfide to allene andsubstituted allene compounds is rapid with at least about 25% andusually about 70 to 100% conversion of the allene compound beingachieved in about 1 to 15 hours. This result is unexpected in view ofthe low reaction rates and conversions obtained in the prior artcondensation of allene and hydrogen sulfide as disclosed in theabovementioned U.S. Pat. No. 3,488,270.

In carrying out the reaction of hydrogen sulfide and an allene compoundaccording to the invention, a photosensitizing compound such asacetophenone, benzophenone and benzaldehyde may advantageously becharged to the reaction mixture in accord with the disclosure of U.S.application Ser. No. 535,632, now U.S. Pat. No. 3,992,274 entitled"Improved sensitized Production of Sulfhydryl Compounds" to B. F.Dannels, filed of even date herewith, the pertinent technology of saidapplication being incorporated herein by reference.

The reaction of hydrogen sulfide and allene compound can be carried outin an appropriate conventional inert solvent e.g. benzene, but ispreferably effected in bulk to facilitate the recovering of thepropanedithiol product. The desired propanedithiol suitable forcondensation with an unsaturated hydrocarbon is readily recovered by aconventional purification technique, advantageously fractionaldistillation of the crude reaction mixture which is preferably carriedout at reduced pressure, typically about 0.1 to 100 mm of mercury. Priorto distillation, the crude is advantageously stripped of volatiles, suchas traces of allyl mercaptan and dissolved hydrogen sulfide by moderateheating, say at 90°-110° Centigrade, at diminished pressure, e.g. about1 to 100 mm, for about 0.5 to 1.5 hours.

In a preferred embodiment of the invention the colorless1,3-propanedithiol or substituted 1,3-propanedithiol is polymerized byreaction with an unsaturated organic compound to provide a substantiallycolorless high molecular weight sulfhydryl-terminated polythioethercapable of being cured to an elastomer. The polymerization reactionproceeds by a free radical mechanism and can be initiated byconventional initiators such as cumene hydroperoxide, tertiarybutylhydroperoxide, azo-bis-isobutyronitrile,azo-bis-2,4-dimethylvaleronitrile, gamma radiation and broad spectrumultraviolet light, i.e. ultraviolet light of 100-3800 Angstromswavelength. Preferably, however, the polymerization is initiated withultraviolet light of wavelength substantially above about 2600Angstroms, i.e. low energy ultraviolet light of the type employed in theabove described reaction of the the allene compound and hydrogensulfide.

The unsaturated compound which is reacted with the sulfhydryl-terminatedproduct of the reaction of hydrogen sulfide and the allene is ahydrocarbon having at least one acetylene bond or at least two ethylenicbonds. Olefins with one ethylenic bond react but do not give polymericproducts. Generally the acetylenically unsaturated reactant will contain2 to 50 carbon atoms and can be of mono, di, tri or higherfunctionality, i.e. it can contain one, two, three or more unsaturatedsites per molecule. The ethylenically unsaturated reactant will contain3 to 50 carbon atoms and be of di, tri, or higher functionality. Ifdesired a multifunctional unsaturated reactant containing both ethylenicand acetylenic unsaturation can be used. The unsaturated reactant maycontain one or more halogen substituents, e.g. fluorine, chlorine, andbromine, attached either to a saturated or unsaturated carbon atom. Alsothe unsaturate may contain other inert substituents such as ethergroups, aryl substituents such as phenyl, naphthyl and loweralkylphenyl, cycloalkyl substituents such as cyclopentyl, cyclohexyl andcyclododecyl and ester groups, i.e. carboxy-lower alkyl groups such ascarboxymethyl. Cyclic, straight-chain and branched chain unsaturatedcompounds can be employed in the chain extension reaction.

Typical suitable unsaturated compounds for use in the invention includethe following representative examples: allene and the above-indicatedsubstituted allenes, cyclohexadiene-1,3 butadiene-1,3, pentadiene-1,3,pentadiene-1,4, hexadiene-1,5, hexadiene-1,4, 2-methylbutadiene-1,3,2,4-dimethylhexadiene-2,4, acetylene, methyl acetylene, butyne-1,butyne-2, bis(pentacosyl)acetylene, pentyne-2, hexyne-1, cetyne-1,octylacetylene, phenylacetylene, cyclopentadiene, 1,3,5-hexatriene,cyclopentylacetylene, 1,2-divinyl cyclohexane,1,3,5-trivinylcyclohexane, 1,2, 4-trivinylcyclohexane, p-divinylbenzene,sym.-trivinylbenzene, unsaturated organic halides such asperfluorobutadiene-1,3, 2-chloromethylbutadiene-1,3, 2-bromomethylpentadiene-1,3, perfluoro-propyne-1, and unsaturated ethers such asdivinyl ether, diallyl ether, dimethallyl ether as well as the homologsand isomers of such compounds. Preferred unsaturated compounds containup to 12 carbon atoms.

In order to obtain an elastomer of desirable mechanical properties forsealant applications the unsaturated reactant is preferably predominatlyan acetylenically unsaturated compound, most preferably amonoacetylenically unsaturated compound and especially an aliphaticacetylene containing the unsaturation solely in the alpha or terminalpositions. Use of a monofunctional aliphatic terminally unsaturatedacetylene, e.g. methylacetylene, provides an especially good result. Inorder to provide desirable chain branching in the sulfhydryl-terminatedpolythioether product, the unsaturated reactant also advantageouslycontains a minor amount of a trifunctional unsaturated hydrocarbon,especially 1,2,4-trivinylcyclohexane.

The chain extension reaction can be carried out at temperatures of about-100° C to +175° C, preferably about 0° to 70° C. at pressures rangingfrom about 1 to about 10 atmospheres, preferably from about 1 to 5atmospheres. The reaction can be effected wholly in the liquid phase ifdesired, employing sufficient pressure to maintain the reactionconstituents in the liquid state. However, when the unsaturated reactantis a gas at the particular reaction temperature used, it is advantageousto bubble or sparge the unsaturated reactant into the liquid reactionmass maintained at atmospheric pressure.

The molar ratio of unsaturated reactant(s) to sulfhydryl-terminatedreactant charged to the polymerization reaction is generally about 0.7:1to about 0.999:1. Molar ratios of unsaturated reactant to dithiolreactant of exactly 1:1 or greater can be used but are desirably avoidedsince such ratios provide polythioether products which terminate inunsaturated substituents rather than in sulfhydryl groups. Preferablythe molar ratio of unsaturated compound(s) to sulfhydryl-terminatedreactant is about 0.85 to about 0.99.

When, in accordance with an especially preferred embodiment of theinvention, the unsaturated reactant contains a monofunctional acetyleniccompound together with a small amount of tri-unsaturated compound, toeffect desirable chain branching in the product, the proportion oftrifunctional unsaturated compound charged is about 0.0005 to 0.05,preferably about 0.005 to 0.03 moles per mole of thesulfhydryl-terminated reactant, i.e. the dithiol.

The polymerization reaction is carried out in reaction equipmentsubstantially similar to that employed in carrying out the reaction ofhydrogen sulfide and the allene compound. When ultraviolet light ofwavelength substantially above about 2600 Angstroms is used to initiatethe polymerization according to a preferred mode of operation, thesulfhydryl-terminated polyethioether product is of substantially greaterpurity and higher molecular weight than the polythioether obtained inthe comparable polymerization initiated with ultraviolet lightcontaining the short wavelength, high energy ultraviolet radiation, i.e.ultraviolet light filtered through quartz or Vycor which containsultraviolet light of wave lengths within the range of about 100 to about2600 Angstroms. For example, preferred ultraviolet light initiation ofthe polymerization reaction results in a colorless sulfhydryl-terminatedpolythioether having a molecular weight of about 10,000 or greater afteronly a five hour reaction period, whereas the polymerization reactioninitiated by ultraviolet light containing the short high energywavelengths results in a polythioether of a molecular weight of only1940 even though the reaction period is extended to 10 hours.Furthermore, the latter reaction mixture is contaminated with anintractable gel which forms on the reaction vessel walls and is, inpart, discolored. The foregoing result also indicates a rapidity ofreaction when employing the preferred ultraviolet light initiation whichis unexpected in view of the lower energy of the initiating ultravioletlight. Generally when initiated with the preferred wavelengths ofultraviolet light, the polymerization reaction mixture reaches itsmaximum viscosity (indicative of maximum product molecular weight) inonly about 1 to 15 hours.

Advantageously, the polymerization reaction may be carried out in thepresence of a photosensitizing compound such as is described above forthe reaction of the allene compound and hydrogen sulfide.

The polymerization can be carried out in a conventional inert solvent orin an appropriate conventional water-surfactant emulsion system.Preferably the polymerization is conducted in bulk which permits directrecovery of the polythioether product without further purificationsteps. The sulfhydryl-terminated polythioether product is fluid andusually can be poured or decanted from the reaction vessel on completionof the reaction. Polymeric products of molecular weight of 20,000 orgreater may be so viscous as to require extrusion from the reactionvessel.

The crude sulfhydryl-terminated polythioether is advantageously strippedof volatiles, e.g. low molecular weight cyclic polythioethers, whichhave an unpleasant odor, by heating the polymer product at about170°-200° Centrigrade under a diminished pressure of about 0.001 to 5 mmof mercury for about 0.5 to 5 hours. Alternatively, the low molecularweight cyclic polythioethers can be removed by conventional steamdistillation.

The sulfhydryl-terminated polythioether which is recovered from thepolymerization reaction of the invention is substantially colorless and,hence, of enhanced commercial value. This class of polythioether isimportant as reactive precursors, i.e. prepolymers, of architecturalsealant elastomers, since they can be cross-linked to elastomericthree-dimensional networks using a variety of known curing techniques.Advantageously these sulfhydryl-terminated polythioethers can be curedby mixing about 1 to 20 grams per 100 grams of polythioether of anoxidizing agent such as lead dioxide, manganese dioxide, and calciumperoxide and allowing the mixture to cure at room temperature.Alternatively, the curing can be accomplished by mixing thesulfhydryl-polythioether with an epoxide compound containing at leasttwo epoxy substituents per molecule and heating the resultant mixtureuntil the desired three dimensional network is obtained. Also thesulfhydryl-terminated polythioethers can be cured by reaction withdiepisulfides and the like.

In using the sulfhydryl-terminated polythioethers as sealants, variousconventional adjuvants are normally added to the polythioether prior toinitiation of the curing operation, for example stabilizers,plasticizers and various types of fillers. Typically carbon fillers suchas carbon black, channel black, furnace or petroleum coke can beincorporated into the sulfhydryl in amounts up to about 200 parts fillerper 100 parts by weight of polythioether. Mineral fillers can be usedalso including the usual non-carbon fillers of pigments such as titaniumdioxide or the oxides, hydroxides, sulfides, carbonates, etc. ofsilicon, aluminum, magnesium, zinc, calcium, barium or the like, as wellas the silicates or aluminates of the various metals indicated above.The production and curing of polythioether elastomeric sealantcompositions from sulfhydryl-terminated polythioethers is moreparticularly described in aforementioned U.S. application S.N. 501,716,the pertinent technology of which is incorporated herein by reference.

The present sulfhydryl-terminated polythioethers are capable of in situroom temperature curing. Moreover, the resultant sealant compositionsare resistant to degradation by oxygen, ozone and organic solvents.Accordingly such sealants are especially useful in architecturalapplications, for example for filling and sealing joints, and seams inmany areas of construction including insulated glass windows, floors andpavements. A particular advantage of the present sulfhydryl-terminatedpolythioethers in such architectural sealant applications is theirsubstantial lack of color. Thus, the present substantially colorlesssulfhydryl-terminated polythioether products can be cured by a white orcolorless curing agent, e.g. calcium peroxide, to provide an attractiveseal devoid of discoloration.

In addition to the foregoing commercial applications, the elastomers andrubbers prepared by curing the sulfhydryl-terminated polythioethers canalso be employed as printing rollers and gaskets.

The following examples serve to illustrate the various aspects of theinvention but are not intended to limit it. Where not otherwise notedthroughout this specification and claims, parts, percentages andproportions are by weight and temperatures are in degrees centigrade.

EXAMPLE 1

A 1.5 gallon stainless steel pressure vessel with a nickel head, and a55 mm outside diameter quartz light well is equipped with pressuresensors, a thermocouple and a Chemical Pyrex (7740 Pyrex manufactured bythe Corning Glass Co.) filter inserted in the light well. A 450 wattmedium pressure mercury vapor arc (Hanovia Model 679A-36 ultravioletlamp) is placed within the light well through which nitrogen gas coolantis circulated. The reaction vessel is cooled to about -20° in an aqueousethylene glycol bath. Hydrogen sulfide and allene in a mole ratio ofabout 8.3:1 are charged to the vessel which is sealed.

The mercury vapor lamp is switched on and allowed to irradiate thereaction mass for about 14 hours during which the reaction pressure isabout 180 to 235 psi. The reaction vessel is then vented to a receiverat atmospheric pressure to recover excess hydrogen sulfide and anyunreacted allene. There is thus obtained 1790 grams, (corresponding toabout 100% conversion based on allene) of clear colorless liquid productdevoid of any suspended or precipitated solid. Vapor phasechromatographic analysis of the crude colorless reaction mixtureindicates the presence of about 68.7 percent of propanedithiol (of whichno more than 5 percent is 1,2-propanedithiol, the remainder being1,3-propanedithiol) with the remaining portion of the crude reactionmixture being a mixture of sulfhydryl-terminated thioethers containing1,2 or more thioether linkages per molecule. Traces of allyl mercaptanand dissolved hydrogen sulfide are removed from the reaction mixture byheating the latter at about 100° at 25 mm of mercury for about one hour.Fractional distillation of the crude reaction mixture through a packedcolumn (16 inch length) provides a colorless fraction, b.p. 87-89/46 mmof mercury, amounting to about 50% of the distillation charge which isidentified as 1,3-propanedithiol containing a trace of the corresponding1,2-propanedithiol. The distillation residue is also colorless.

Substantially similar excellent clear colorless 1,3-propanedithiol canbe obtained by replacing the Pyrex light well in the above reaction withone of Corex (9700 Corex manufactured by the Corning Glass Co.).

The composition of 7740 Pyrex glass is 81% SiO₂, 13% B₂ O₃, 4% Na₂ O,and 2% Al₂ O₃.

The composition of 9700 Corex glass is 80% SiO₂, 13% B₂ O₃, 5% Na₂ O and2% Al₂ O₃.

EXAMPLE 2

A 0.5 liter Vycor (7910 Vycor glass manufactured by Corning Glass Co.)round bottomed flask fitted with a stirrer, thermometer, gas inlet tubeand dry ice reflux condenser is flushed with nitrogen. The flask ischarged with 108 grams (about one mole) of distilled colorless1,3-propanedithiol obtained by a procedure substantially similar to thatof Example 1, and 4.1 grams (0.026 mole) of 1,2,4-trivinylcyclohexane. A450 watt medium pressure mercury arc lamp of the type employed inExample 1 positioned horizontally 4 inches beneath the flask is allowedto illuminate the stirred reaction mixture for thirty minutes, then thereaction mass is saturated with gaseous methylacetylene charged throughthe gas inlet tube beneath the surface of the reaction mass which ismaintained at a temperature of about 25°-30° at atmospheric pressure.The methylacetylene addition is continued at a rate sufficient tomaintain slight reflux in the reaction mixture. After 10 hours, 34 grams(0.85 moles) of methylacetylene has reacted. The liquidsulfhydryl-terminated polythioether (146.3 grams) which is decanted fromthe reaction flask is heated at about 200° at a diminished pressure ofabout 0.5 mm of mercury for about one hour to remove volatiles ofunpleasant odor. The product has a viscosity of 348 poises and a numberaverage molecular weight of 1940 (vapor pressure osmometer intetrahydrofuran).

The sulfhydryl polythioether product is colorless but cloudy and can becured at room temperature to an attractive, substantially colorlesspolythioether elastomer with a suitable curing agent such as calciumperoxide in accord with the procedure of aforementioned U.S. applicationNo. 501,716.

An intractable gelled material is found adhering to the walls of thereaction flask after removal of the sulfhydryl-terminated polythioether.While the gelled by-product is in the main colorless, that portion onthe flask wall nearest the ultraviolet source has brown discoloration.

A substantially similar result is obtained when the above describedpolymerization reaction is carried out in a quartz flask, since quartz,like Vycor, is transparent to ultraviolet light of wavelength belowabout 2600 Angstroms.

EXAMPLE 3

The procedure of Example 2 is repeated substantially as described exceptthat the ultraviolet lamp is positioned two inches from the flask whichis of Chemical Pyrex rather than Vycor and 87 grams (0.81 mole) of thedistilled colorless 1,3-propanedithiol and 3.18 grams (0.02 mole) of the1,2,4-trivinylcyclohexane are charged. After about 5 hours reaction 30.8grams (0.75 moles) of methylacetylene react and the recoveredvacuum-stripped sulfhydryl-terminated polythioether is clear, colorlessand very viscous (about 2000 poise viscosity or greater, indicative of aproduct molecular weight of 10,000 or greater). No gelled material isformed. The clear colorless sulfhydryl-terminated polythioether can becured at room temperature to an attractive, substantially colorlesspolythioether elastomer by the procedure of Example 2.

Substantially similar excellent results are obtained when the pyrexreaction flask is replaced by one of Corex 9700, or ordinary soda limeglass.

Comparison of the results of Examples 2 and 3 indicate that thepolymerization reaction is preferably initiated with ultraviolet lighthaving a wavelength substantially above about 2600 Angstroms.

EXAMPLE 4

The procedure of Example 1 is repeated substantially as described exceptthat the molar ratio of hydrogen sulfide to allene is 9.3:1. After a 9hour reaction period there is obtained 1090 grams (corresponding to 71%conversion of allene) of clear colorless reaction mixture which containsabout 66 mole percent of propanedithiol. Fractional distillation underdiminished pressure of the reaction mixture provides a clear colorless1,3-propanedithiol distillate and colorless distillation residue.

The distilled 1,3-propanedithiol product can be reacted withmethylacetylene and 1,2,4-trivinylcyclohexane in accord with theprocedure of Example 3 to obtain a commercially attractive colorlesssulfhydryl-terminated polythioether curable to an attractive colorlesspolythioether elastomeric sealant.

EXAMPLE 5 (CONTROL)

The procedure of Example 1 is repeated substantially as described exceptthat the molar ratio of hydrogen sulfide to allene is about 8.7:1 andthe Pyrex insert is omitted so that the quartz light well transmits tothe reaction mixture ultraviolet light of wavelengths below about 2600Angstroms as well as wave lengths of ultraviolet light up to about 3800Angstroms. After an 11.2 hour reaction period, there is obtained 1213grams (corresponding to a 72% conversion of allene) of crude reactionmixture which is yellow and cloudy indicating the presence of suspendedsolid, which is identified as sulfur. Analysis of the reaction massindicates the presence of about 58 mole percent of propanedithiol.Fractional distillation of the reaction mixture provides a 46% recovery(based on the distillation charge) of a 1,3-propanedithiol fraction,b.p. 84-87 at 39 mm of mercury, which is yellow but free of suspendedsulfur. Neither refractionation nor absorption treatment, (on a verticalcolumn filled with particulate absorbent alumnia) removes the yellowcolor of the distilled 1,3-propanedithiol.

The discolored, distilled 1,3-propanedithiol is reacted withmethylacetylene and 1,2,4-trivinylcyclohexane substantially in accordwith the procedure of Example 3. The resultant sulfhydryl polythioetheris yellow and on being cured with a colorless curing agent provides anunattractive yellow polythioether elastomer.

A substantially similar undesirable result, i.e. a yellow propanedithiolproduct and formation of elemental sulfur, is also obtained when theabove reaction of allene and hydrogen sulfide is carried out in areactor equipped with a Vycor light well filter insert.

From comparison of the results of Examples 1 and 4 with that of controlExample 5 it is apparent that initiation of the allenehydrogen sulfidereaction with ultraviolet light of wavelength substantially above about2600 Angstrom not only provides a dithiol and polymerization productdevoid of color but also avoids formation of elemental sulfur.

EXAMPLE 6

The procedure of Example 3 is repeated substantially as described inpolymerizing 114 grams (about 1.05 mole) of fractionally distilled1,3-propanedithiol prepared as described in Example 1 with 0.86 grams(0.0053 mole) of 1,2,4 trivinylcyclohexane and 41.7 grams (1.04 mole) ofmethyl acetylene except that the polymerization is carried out in thepresence of 0.53 grams (0.0044 mole) of acetophenone charged as aphotosensitizing reagent and the ultraviolet light source is a 100 wattultraviolet light positioned 1/2 inch beneath the pyrex flask. After areaction period of about 5 hours there is obtained a liquid clear,substantially colorless sulfhydryl-terminated polythioether which, afterbeing stripped of volatiles by heating at 200° for one hour at adiminished pressure of 0.05 mm of mercury, has a viscosity of about20,000 poises and a molecular weight of about 16,800 (gel permeationchromatography). The sulfhydryl-terminated polythioether is cured to anexcellent polythioether elastomer by a procedure substantially similarto that of Example 2. The foregoing procedure can be repeated to obtainsulfhydryl-terminated polythioethers of hig molecular weight, e.g. ofabout 100,000 to 1,000,000 or higher.

It is to be understood that the invention is not limited to the specificexamples which have been offered merely as illustrative and thatmodifications may be made without departing from the spirit of theinvention.

What is claimed is:
 1. In a process for the formation of asulfhydryl-terminated polythioether which comprises the steps of1.Condensing an allene compound and hydrogen sulfide in liquid phase toproduce a sulfhydryl-terminated hydrocarbon,
 2. Recovering thesulfhydryl-terminated hydrocarbon,
 3. Condensing the recoveredsulfhydryl-terminated hydrocarbon with an unsaturated hydrocarbon havingat least one acetylenic bond or at least two ethylenic bonds to obtainthe sulfhydryl-terminated polythioether,the improvement which comprisescarrying out the step 1 reaction at a reaction temperature of about-100° to about 95° Centigrade, a reaction pressure of about 0 to about3000 p.s.i.g. and a molar ratio of hydrogen sulfide to the allenecompound of about 1:1 to about 20:1 in the presence of ultraviolet lightof wavelength substantially above about 2600 Angstroms to obtain asulfhydryl-terminated polythioether substantially devoid of color. 2.The process of claim 1 wherein the allene compound is one wherein theallene linkage is terminal, the reaction temperature of step 1 is about-70° to about 90° Centigrade, the reaction pressure of step 1 is about 0to 1300 p.s.i.g. and the molar ratio of hydrogen sulfide to the allenecompound in the reaction of step 1 is about 1.5:1 to about 15.1, thereaction temperature of step 3 is about -100° to about 175° Centigrade,the reaction pressure of step 3 is about 1 to about 10 atmospheres, andthe molar ratio of the unsaturated hydrocarbon reactant to thesulfhydryl-terminated hydrocarbon reactant in step 3 is about 0.7:1 toabout 0.999:1.
 3. The process of claim 2 wherein the allene compound isa lower aliphatic allene and the molar ratio of hydrogen sulfide to theallene compound is about 2:1 to about 10:1 and the unsaturatedhydrocarbon reactant in step 3 contains 2 to 50 carbon atoms.
 4. Theprocess of claim 3 wherein the allene compound is allene, thesulfhydryl-terminated hydrocarbon product is 1,3-propanedithiol, theunsaturated reactant contains 2 to 12 carbon atoms, and the reaction ofthe propanedithiol and the unsaturated reactant in step 3 is initiatedby ultraviolet light of wavelength substantially above about 2600Angstroms.
 5. The process of claim 4 wherein the unsaturated hydrocarbonreactant of step 3 is predominately an acetylenic compound and thereaction of the propanedithiol and the unsaturated hydrocarbon in step 3is effected at a pressure of about 1 to 5 atmospheres and at atemperature of about 0° to about 70° Centigrade.
 6. The process of claim5 wherein the predominant acetylenic reactant is mono-unsaturated, themolar ratio of said hydrocarbon to 1,3-propanedithiol is about 0.85:1 to0.99:1 and the ultraviolet light of wavelength substantially above about2600 Angstroms in steps 1 and 3 is provided by passing ultraviolet lightthrough vitreous material which is transparent to at least somewavelengths of light in the 2600-3800 Angstrom wavelength region but issubstantially opaque to light in the 100-2600 wavelength region.
 7. Theprocess of claim 6 wherein the predominant acetylenic reactant is aterminally unsaturated aliphatic acetylene compound and the unsaturatedreactant also include about 0.0005 to about 0.05 mole per mole of thepropanedithiol reactant of a tri-unsaturated hydrocarbon.
 8. The processof claim 7 wherein the acetylene compound is methylacetylene, thetri-unsaturated hydrocarbon reactant is an ethylenically unsaturatedhydrocarbon charged in a concentration of about 0.005 to about 0.03 moleper mole of the propanedithiol.
 9. The process of claim 8 wherein thevitreous filter is Chemical Pyrex.
 10. The process of 8 wherein thevitreous filter is Corex
 9700. 11. The process of claim 1 wherein therecovery of the sulfhydryl-terminated hydrocarbon is carried out byfractional distillation.
 12. In a process for the formation of asulfhydryl-terminated polythioether which comprises the steps of1.Condensing an allene compound and hydrogen sulfide in liquid phase toproduce a sulfhydryl-terminated hydrocarbon,
 2. Recovering thesulfhydryl-terminated hydrocarbon,3. Condensing the recoveredsulfhydryl-terminated hydrocarbon with a hydrocarbon having at least oneacetylenic bond or at least two ethylenic bonds to obtain thesulfhydryl-terminated polythioether,the improvement which comprisescarrying out steps 1 and 3 in the presence of ultraviolet light ofwavelength substantially above about 2600 Angstroms with the step 1reaction temperature being about -100° to about 95° Centigrade, the step1 reaction pressure being about 0 to about 3000 p.s.i.g., the molarratio of hydrogen sulfide to the allene compound in step 1 being about1:1 to about 20:1, the step 3 reaction temperature being about -100° toabout 175° Centigrade, the step 3 reaction pressure being about 1 toabout 10 atmospheres and the molar ratio of the unsaturated hydrocarbonreactant to the sulfhydryl-terminated hydrocarbon reactant in step 3being about 0.7:1 to about 0.999:1, whereby the reactions of steps 1 and3 are effectively initiated and the purity of the sulfhydryl-terminatedpolythioether is enhanced.
 13. In the process for the formation of asulfhydryl-terminated polythioether which comprises polymerizing anunsaturated hydrocarbon having at least one acetylenic bond or at leasttwo ethylenic bonds with a dithiol selected from the group consisting of1,2 and 1,3-propanedithiol and substituted derivatives thereof which areprepared by the reaction of an allene and hydrogen sulfide, theimprovement which comprises carrying out the reaction in the presence ofultraviolet light of wavelength substantially above about 2600 Angstromsat a reaction temperature of about -100° to about 175° Centigrade and areaction pressure of about 1 to about 10 atmospheres and at a molarratio of the unsaturated hydrocarbon reactant to the dithiol of about0.7:1 to about 0.999:1 whereby the reaction is effectively initiated andthe purity of the sulfhydryl-terminated polythioether is enhanced.